This invention relates to radio communication systems employing antenna arrays and has particular application to multi-carrier communications systems such as those employing code division multiple access (CDMA) techniques.
One type of communications system is a cellular communications system. In a cellular communications system, a plurality of base stations provides a radio communications service to a plurality of remote mobile subscriber units known as mobile stations. Each base station defines a particular geographical area or cell proximate to the base station to produce coverage areas. The communications link from the base station to a mobile station is referred to as the downlink. Conversely, the communications link from a mobile subscriber unit to the base station is referred to as the uplink.
Multiple access techniques such as CDMA, permit simultaneous transmission between several mobile stations and a single base station.
CDMA employs spread spectrum signaling whereby individual users in the communications network use the same RF carrier frequency, but are separated by the use of individual spreading codes. Hence, multiple communications channels are allocated using a plurality of spreading codes within the portion of radio spectrum, each code being uniquely assigned to a mobile station.
In some systems, one carrier frequency is allocated for communication on the downlink and another carrier frequency is allocated for communication on the uplink. This is known as FDD (frequency division duplex).
Another system uses a method known as time division duplex (TDD) in which the same carrier frequency is used for both uplink and downlink. A repeating time-frame structure is, in this case, divided into an interval with time slots used in the uplink direction and another interval with time slots used in the downlink direction.
Some systems are known which utilise knowledge of the propagation characteristics of the communication channel to adjust a characteristic of the transmission in order to achieve a more efficient use of the available resources. Most of these systems use feedback of information on the state of the link. Some systems which operate in TDD mode, use a calculated estimate of the propagation channel characteristics in one direction for adjusting a characteristic of the transmission in the other direction. Thus a base station can derive an estimate of the characteristics of the downlink propagation channel from received uplink measurements.
A base station may be fitted with single antenna or with an array of antenna elements.
Antenna arrays can provide improved performance relative to a single antenna by providing a better antenna pattern for a coverage area.
Even with an antenna array to provide an improved antenna pattern, signals communicated between the base stations and mobile stations are subject to interference. Buildings, hills and other objects produce multi-path wave propagation resulting in errors in the signals communicated.
To reduce these errors, receive and transmit diversity schemes have been developed to optimise the receive and transmit path of communications systems employing antenna arrays. By varying the weight of the signals detected by each of the individual antennas in the array, it is possible to vary the antenna pattern to better detect signals from a particular direction or to arrange for non-destructive combination of multi-path signals. These techniques adjust the weights of the antenna array signals to maximise the receive path gain by measuring the output of a receiver.
Other techniques are known whereby optimum weights are provided for the transmit path. For example, Applicants co-pending Application number GB-A-2313261 discloses a method of weighting a transmit path of a communication station A which is equipped with an antenna array. A method includes the steps of transmitting reference signals from each antenna array to a communications station B and calculating at station B weighting information based on a comparison of the incoming reference signals with stored signals. The calculated weighting information is then transmitted from station B to station A whereupon a controller in station A adjusts the antenna weights based on the received weighting information.
Transmit diversity schemes can significantly improve the downlink performance of the so-called third generation systems or UMTS (universal mobile telecommunications system) which employ CDMA techniques. However, for a CDMA system operating in time duplex mode (TDD) (where the same carrier frequency is used for both uplink and downlink), this leads to a significant increase in complexity of channel estimation at the mobile station.
The UMTS TDD mode uses TD-CDMA as multiple access technology. In this system, mobile stations and base stations use interference cancellation techniques or joint detection to decode mobile stations accessing the channel at the same time. Each user""s mobile station in a cell receives the downlink signal of all the mobile stations accessing the same time slot/frequency. All this information is transmitted over the same propagation channel. Therefore, only one channel estimation is required at the mobile station.
If transmit adaptive antennas or selection transmit diversity are implemented at the base station on a per user basis, the information intended for each user is modified by a different weight. Therefore, a single mobile station (user) will receive the information in the same slot/frequency as if it had been transmitted over K different channels, K being the number of users accessing the same slot/frequency. As a consequence, the interference cancellation scheme or joint detector will require the estimation of this K different propagation channels. This will lead to a significant complexity increase of the mobile station. In fact, the channel estimation algorithm is approximately 10 times more complex.
The present invention aims to overcome this disadvantage by means of a joint optimisation criteria where the same transmission settings are applied for all mobile stations.
Accordingly, the present invention consists of a communications system including a base station for communicating with at least first and second mobile stations over respective first and second propagation channels, in which the base station includes at least first and second antennas and in which the system further includes means for deriving an estimate of the characteristics of each propagation channel,
and characterised by a phase adjuster for adjusting a phase difference between communications signals transmitted by the antennas (2,3),
and a control unit (9) for computing the product of the estimates,
in which the phase adjuster (10) is adapted to adjust said phase difference until said product attains a maximum value.
In one embodiment, eg. TDD mode, the channel estimates are computed at the base station and derived from uplink measurements received from each mobile station.
In an alternative embodiment, eg.FDD mode, the channel estimates are computed at each mobile station and fed back to the base station.
If all the users are optimised at the same time (i.e. the phase alignment transmit diversity scheme offered by the present invention is used and in addition, only one weight is applied at the transmitter), a single mobile station will see the received information as if it has been transmitted over a single propagation channel (i.e the real channel multiplied by the applied weight). Therefore, independently of the number of users, only one channel will have to be estimated and the implementation of transmit diversity at a base station will not increase the complexity at a mobile station.